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1
Clean the portobellos with a dampened paper towel. Remove the stems and mince them.
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2
Lightly coat medium saute pan with a film of olive oil. Add leeks and the minced mushroom stems and saute over medium heat until wilted, about 10 minutes. Add spinach, garlic and dill and season with a VERY light amount of salt (remember, the feta will add more salt later) and pepper. Cook a further 5 minutes. Remove from heat and cool. While cooling make bechamel sauce.
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3
Preheat oven to 410F (210C). Oil a pan large enough to hold the mushrooms.
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4
Bechamel Sauce: Melt butter in small saucepan over medium-low heat. When melted, add flour and cook, whisking constantly, for 2-3 minutes. If flour looks like it's starting to brown, remove from heat and continue whisking for time stated. Add milk, a dash of salt to taste and nutmeg. Bring to a boil, whisking constantly, and boil for 2 minutes, until thick.
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5
Add 4 tblsps of the bechamel sauce to the spinach mixture, along with the beaten egg and the crumbled feta and mix well.
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6
Brush the mushrooms with oil and fill the cavities with equal amounts of the spinach filling, flattening the tops of the mounds slightly. Give the bechamel sauce a whisk if it has started to develop a 'crust' and dollop equal amounts of the bechamel over each mushroom (do not worry if the bechamel doesn't dribble down sides at this point - it will during baking).
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7
Place mushrooms in bottom third of preheated oven and bake for 30 minutes. The bechamel should start to get brown patches on it - if it's browning too quickly, loosely cover with aluminum foil and continue baking.
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8
Enjoy!
